Highly ion-conductive gel polymer electrolyte (GPE) with mechanical flexibility is developed by incorporating liquid electrolyte into polymer films that are fabricated by initiator-free photopolymerization of poly(ethylene glycol) dimethacrylate (acrylate monomer) and pentaerythritol tetrakis (3-mercaptopropionate) (thiol monomer) blend. When UV is irradiated on the blend, the thiol monomers themselves produce radicals to initiate the polymerization. GPEs with 40–50 wt.% of thiol monomer content show mechanically free standing characteristics with sufficient flexibility. The ionic conductivity of the GPE reaches1.1 × 10−3 S cm−1 at 25 °C and is thus high enough to be applied for lithium secondary batteries. The GPE is electrochemically stable up to 4.4 V versus Li/Li+ and the unit cells consisting of LiCoO2/GPE/lithium metal show good cycling performance. This GPE is thus considered a good electrolyte candidate for future flexible and wearable lithium secondary batteries.